This research proposal is designed to further develop a unique model system to study the interactions of steroid hormones in target cells and the biochemical mechanism of hormone action. Antheridiol is a steroid hormone which induces male sexual differentiation in the oomycete fungus, Achlya ambisexualis. This water mold is the most primitive eukaryotic organism known in which sexual reproduction is controlled by a steroid hormone system. It offers several advantages for investigations on hormone action due to its relatively simple cellular organization and reproductive system. Over the past five years, we have synthesized a radioactive steroid that has been used to identify antheridiol receptors in Achlya. The major receptor component in Achlya cytosol was partially characterized and shown to be remarkably similar to steroid receptors in higher organisms. We propose to pursue this system along two major lines of investigation. The molecular properties of the antheridiol receptor will be described in greater detail. To achieve this, highly purified receptor preparations will be obtained and monoclonal antibodies to the receptor will be prepared as necessary tools for further analysis. Properties to be analyzed include molecular weight, subunit and amino acid composition, and receptor phosphorylation. The probable association of the receptor with another cellular protein will be documented and described in more detail. In addition, studies will be initiated on the steroid hormone oogoniol, which directs sexual morphogenesis of the female. A suitable radioactive steroid will be prepared for the detection and analysis of oogoniol receptors in Achlya. These studies should provide some very important tools for the development of this experimental model. Comparisons will be made with higher organisms which should reveal some of the most general and important features of steroid regulation in eukaryotes.